Natural gas-fired engines are widely used in the petroleum industry typically to drive compressors that compress natural gas at well heads and along pipelines. In other industries they are often used for in-house electric generators and co-generation systems. In general these gas fired engines are designed to operate at higher temperatures than other internal combustion engines. Additionally these engines are operated near full load conditions for significant time periods, if not continuously. Under these service conditions the life of gas engine lubricants is often limited by oil oxidation and nitration processes. Therefore, gas engine oils are formulated with additives to extend oil life through enhanced resistance to oil oxidation and nitration.
In addition to controlling oxidation and nitration properties of a gas engine oil, it also is necessary to control the ash content of the oil because the ash acts as a solid lubricant protecting, for example, the valve/seat interface of the engine.
The ash level of the lubricant often is determined by its formulation components, with metal-containing detergents and metallic-containing antiwear additives contributing to the ash level of the lubricant. Gas engine manufacturers specify the appropriate lubricant ash level for correct operation of a given engine. Thus, manufacturers of 2-cycle engines often specify use of an ashless oil. Manufacturers of 4-cycle engines may specify low, medium or high ash depending upon the level required for engine cleanliness and durability.
For this reason gas engine oils are classified according to their ash content. The classifications are:
Ash DesignationAsh Level, wt % (ASTM D874)AshlessAsh < 0.1%Low Ash0.1% < Ash < 0.6%Medium Ash0.6% < Ash < 1.5%High AshAsh > 1.5%
A low ash gas engine oil is described, for example, in U.S. Pat. No. 5,726,133 and medium and high ash oils in U.S. Pat. No. 6,191,081.
As is known in the art, additives are used in lubricants to perform numerous functions. For example, some are antioxidants, some are friction modifiers; and some are extreme pressure agents. Indeed some additives perform more than one function. Also as is known in the art, additives will lose their effectiveness if they are improperly combined. Therefore, extreme care must be exercised in combining various additives to assure both compatibility and effectiveness. For example, some friction modifiers affect metal surfaces differently than antiwear agents do. When both are present, friction-reducing and antiwear additives may compete for the surface of the metal parts which are subject to lubrication. This competition can produce a lubricant that is less effective than is suggested by the individual properties of the additive components.
Accordingly, the components of a gas engine lubricant need to be selected to meet the specified ash level and to provide, among other functions, a high level of oxidation and nitration resistance. Whether selected components and their amounts can be balanced to meet desired specification is not a priori predictable.
Many stationary four-cycle gas engines require exhaust catalysts to meet local exhaust emissions limits. Phosphorus emissions the exhaust catalyst material and so manufacturers have placed limits on the fresh oil's phosphorus content. Currently, the strictest limit is 0.03 wt % phosphorus and it is possible that lower phosphorus levels may be legislated in the future. The source of phosphorus in gas engine oils is the ZDDP antioxidant/antiwear additive used in the oil. Reducing ZDDP treats in the oil to lower the phosphorus content is expected to shorten oil life. Therefore, new gas engine oil compositions wit very low phosphorus levels and good antioxidant and antiwear properties are needed.